Distortion

Typical for extreme range zooms the Tamron exhibits a fairly hefty amount of barrel
distortion at 18mm (~4%). However, beyond the wide end the problem is actually
very well controlled with only slight to moderate pincushion distortion.

Move the mouse cursor over the focal length text marks below to observe the respective distortion

Vignetting

The Tamron is a dedicated APS-C lens but despite the ambitious design vignetting
is generally very well controlled except at 18mm @ f/3.5 (~1.2EV) and 250mm @ f/6.3 (~0.7EV).

MTF (resolution)

So far the league of the extreme range zoom lenses didn't really shine regarding
their resolution characteristic but surprisingly the Tamron designers did a pretty
good job here. In the 18-80mm range the center performance is generally excellent
and the borders are on a very good level. The extreme corners are slightly weaker
at 18mm @ f/3.5 and 35mm @ f/4. There's a continuous drop in resolution towards
the long end of the range. At 250mm @ f/6.3 the center is still very good but the
borders start to soften a bit and the overall contrast level is also somewhat
reduced. Stopping down to f/8 boosts the quality to very good figures again.

Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Imatest Explanations

Chromatic Aberrations (CAs)

For most of the zoom range CAs (color shadows at harsh contrast transitions) are fairly
well controlled with an average pixel width between 0.7px to 1.4px at the image borders.
At 250mm the problem gets more pronounced and the CA distribution is no longer symmetrical
at this setting (peaking beyond 2px in one corner).